One of the fundamental rules regarding the organization of the spinal cord is that sensory information enters through the dorsal roots and the ventral roots carry motor commands. This important generalization is challenged recently by the discovery of large numbers of afferent fibers in the ventral root. However, the functional importance of these ventral root afferents is yet to be demonstrated. The long term goal of the present proposal is to demonstrate the importance of the ventral root afferent fibers in sensory functions with special reference to nociception. To accomplish this goal, neurophysiological methods are employed first to demonstrate that a large number of dorsal horn cells in the cat spinal cord can be activated by ventral root stimulation. The sensory capacity of the dorsal horn cells that receive the ventral root input will be characterized in relation to the sensory function of the ventral root afferents. Attempts will be made to determine the peripheral origin of the afferent fibers in the ventral root and the central destination of the dorsal horn cells which are activated by the ventral root afferents by utilizing both anatomical and physiological methods. Studying the responses of the spinal cells to natural stimuli and determining the central destination of the processes of these cells should provide clues as to the sensory function of the ventral root afferents. Finally, the possible nociceptive role of the ventral root afferents will be tested on primate spinothalamic tract cells by observing the responsiveness of these cells to ventral root stimulation. This is because ventral root afferents have been implicated in nociception and the primate spinothalamic tract is the best established nociceptive tract in an animal model. Presumably these studies will have an impact of both basic and clinical significance. For example, elucidation of the organization of a previously unstudied group of primary afferents cannot help but improve our general understanding of sensory organization and such information will be important in evaluating the effects of dorsal rhizotomy and dorsal root ganglionectomy in the treatment of chronic segmental pain, which is so disabling in certain pathologic conditions in humans.